Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-07-10
2002-10-15
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S046000, C257S047000, C257S197000, C257S198000, C438S170000, C438S172000, C438S312000, C438S320000, C438S571000
Reexamination Certificate
active
06465804
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to heterojunction bipolar transistors (HBTs) for high power applications, and more particularly, to a heterojunction bipolar transistor having an emitter structure capable of reducing the current crowding effect and preventing thermal instabilities, wherein a negative differential resistance (NDR) element is added to the layer structure of the conventional emitter. The NDR element is designed to limit the tunneling current to the maximal emitter current density required for safe transistor operation.
BACKGROUND OF THE INVENTION
The effect of current crowding is a major obstacle in the operation of conventional bipolar power transistors. Due to the relatively large sheet resistance of the transistor base layer, the lateral conduction of the base current causes an ohmic voltage drop which reduces the effective forward bias applied to regions of the emitter junction away from the base contact, as described in the paper by J. R. Hauser, “The Effects of Distributed Base Potential on Emitter Current Injection Density and Effective Base Resistance for Stripe Transistor Geometries”,
IEEE Transactions on Electron Devices
, ED 11, pp. 238-242, (1964). As the base current is increased, the lateral voltage drop becomes larger and the non-even distribution of the emitter current is increased.
The high density of the emitter and collector currents near the base contact causes significant localized heating. The elevated temperature causes an increase in the base collector junction leakage current, reduces the base emitter turn-on voltage, as described in the paper by R. H. Winkler, “Thermal Properties of High Power Transistors”,
IEEE Transactions on Electron Devices
, ED14, pp. 260-263, (1967) and modifies the transistor gain. If the transistor current gain changes are small, a positive feedback effect is generated. If the current of the device is not limited by the external circuit, this thermal runaway can destroy the transistor, as described in B. G. Streetman, “Solid State Electronic Devices”, 3rd ed., Prentice Hall, Englewood Cliffs, N.J., pp. 271-272, (1990).
A common solution to the current crowding effect is the fabrication of a multiple finger transistor made of several thin fingers that are connected in parallel. Such devices are used for high power and microwave applications, as described in S. M. Zee, “Physics of Semiconductor Devices”, 2nd ed., Wiley, New York, pp. 165-166, (1981). Another advantage of this configuration is the improved high frequency operation due to the reduced effective base resistance.
However, when high power is applied to a multiple finger transistor the effects of thermal instability are exhibited, due to an uneven distribution of the current between the transistor fingers. These effects are described in the paper by H. F. Chau, W. Liu and E. A. Beam III, “InP Based HBTs and Their Perspective for Microwave Applications”, 7th International Conference on Indium Phosphide and Related Materials, pp. 640-643, (1995), and the paper by W. Liu, H. F. Chau and E. Beam III, “Thermal Properties and Thermal Instabilities of InP Based Heterojunction Bipolar Transistors”,
IEEE Transactions on Electron Devices,
43, pp. 388-395, (1996). Also, the effect of the current gain collapse takes place, as described in the paper by W. Liu, S. Nelson, D. G. Hill and A. Khatibzadeh, “Current Gain Collapse in Microwave Multifinger Heterojunction Bipolar Transistors Operated at Very High Power Densities”,
IEEE Transactions on Electron Devices,
40, pp. 1917-1927, (1993), and the paper by W. Liu and A. Khatibzadeh, “The Collapse of Current Gain in Microwave Multi-finger Heterojunction Bipolar Transistors: Its Substrate Temperature Dependence, Instability Criteria and Modeling”,
IEEE Transactions on Electron Devices,
41, pp. 1698-1707, (1994).
The thermal instability occurs since the base collector leakage current increases and the transistor turn-on voltage decreases with the rise in temperature. The hottest finger gradually dominates the device operation by drawing higher portion of the total current. The increased current flow causes further heating and a destructive regenerative process is generated.
This thermal instability can be stabilized at the cost of an increased emitter resistance, by adding a stabilizing ballast resistor to each of the emitter fingers, as mentioned in the above-referenced book by S. M Zee (pp. 169-170). Another approach is the incorporation of a high resistance n-type layer in the emitter, as described in the patent by W. Liu and D. G. Hill, “Microwave heterojunction bipolar transistors with emitters designed for high power applications and method for fabricating same” European patent EP 0562272, (1993). It is important to note that high emitter resistance deteriorates the microwave performance of the device.
While it has been previously proposed to integrate a Resonant Tunnel Diode (RTD) in the emitter of bipolar transistors, for the purpose of multilevel logic design, its application in reducing the current crowding effect and preventing thermal instabilities has not been previously discussed. The known applications of RTD in a multilevel logic design are described in the paper by F. Capasso, S. Sen, A. Y. Cho and D. L. Sivco, “Multiple Negative Transconductance and Differential Conductance in a Bipolar Transistor by Sequential Quenching of Resonant Tunneling”, Applied Physics Letters, 53, pp. 1056-1058, (1988), also in the paper by S. Sen, F. Capasso, A. Y. Cho and D. L. Sivco, “Multiple-State Resonant-Tunneling Bipolar Transistor Operating at Room Temperature and its Applications as a Frequency Multiplier”,
IEEE Electron Device Letters,
9, pp. 533-535, (1988), and in the paper by F. Capasso, S. Sen, F. Beltram, L. M. Lunardi A. S. Vengurlekar, P. R. Smith, N. J. Shah, R. J. Malik and A. Y. Cho, “Quantum Functional Devices: Resonant Tunneling Transistors, Circuits with Reduced Complexity, and Multiple-Valued Logic”,
IEEE Transactions on Electron Devices,
36, pp. 2065-2082, (1989).
Therefore, it would be desirable to provide a solution to the operational problems of high power bipolar transistors which are related to the current crowding effect and thermal instabilities.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to overcome the problems associated with the high power operation of bipolar transistors, by proposing and demonstrating a modified emitter structure which includes a Negative Differential Resistance (NDR) element added to the emitter layers. The NDR element limits the emitter current density, reduces the current crowding effect in a large area device and prevents the thermal instabilities in multiple finger transistors. The modified emitter design demonstrates the reduction of the current crowding effect in a large area device and the self current limiting property of a small area device. Both properties can be used to enhance the high power performance of the transistor and prevent thermal instabilities in a multiple finger transistor configuration.
In accordance with a preferred embodiment of the invention, there is provided a bipolar transistor having improved high power performance by reduction of the current crowding effect, said transistor comprising:
a substrate;
a sub-collector layer;
a collector layer;
a base layer; and
an emitter layer structure comprising a negative differential resistance element.
In a preferred embodiment, a resonant tunnel diode (RTD) or an Esaki diode is used as the negative differential resistance element. The maximal density of the forward tunneling current in a resonant tunnel diode (RTD) or an Esaki diode is determined by the device design. When such diode is added to the emitter of a bipolar transistor a composite device is formed where the maximal emitter current density is self-limited.
In a large area transistor, the limit on the emitter current density reduces the current crowding effect. This results from the fact that the peak concentration of the emitter current can not rise beyond t
Ritter Dan
Shamir Nachum
Flynn Nathan J.
Forde Remmon R.
Langer Edward
Technion Research & Development Foundation Ltd.
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